当我们没有权限访问业务表，但是需要查看shared pool中部分sql语句的执行计划，原则上来说，查询v$sql_plan视图结合hash_value可以实现，但是因为这个是表格形式，看起来不太美观，和我们长看的执行计划有一定的出入，这里提供两个脚本，实现查看该种情况下的执行计划。
oracle 9i

[oracle@xifenfei ~]$ more get_plan.sql
set pagesize 0
set linesize 150
set serveroutput on size 10000
col plan_table_output format a125
undefine hash_value
set verify off feedback off
var hash_value varchar2(20)
begin
  :hash_value := '&hash_value';
end;
/
insert into plan_table
      (statement_id,timestamp,operation,options,object_node,object_owner,object_name,
       optimizer,search_columns,id,parent_id,position,cost,cardinality,bytes,other_tag,
       partition_start,partition_stop,partition_id,other,distribution,
       cpu_cost,io_cost,temp_space,access_predicates,filter_predicates
      )
select distinct hash_value,sysdate,operation,options,object_node,object_owner,object_name,
       optimizer,search_columns,id,parent_id,position,cost,cardinality,bytes,other_tag,
       partition_start,partition_stop,partition_id,other,distribution,
       cpu_cost,io_cost,temp_space,access_predicates,filter_predicates
  from v$sql_plan
 where hash_value = :hash_value
/
col piece noprint
select distinct piece,sql_text from v$sqltext where hash_value = :hash_value order by piece
/
@?/rdbms/admin/utlxplp.sql
set linesize 80
set verify on feedback on pagesize 1000

oracle 10g/11g

[oracle@xifenfei ~]$ more get_plan.sql
set pagesize 0
set linesize 150
set serveroutput on size 10000
col plan_table_output format a125
undefine hash_value
set verify off feedback off
var hash_value varchar2(20)
begin
  :hash_value := '&hash_value';
end;
/
insert into plan_table
      (statement_id,timestamp,operation,options,object_node,object_owner,object_name,
       optimizer,search_columns,id,parent_id,position,cost,cardinality,bytes,other_tag,
       partition_start,partition_stop,partition_id,other,distribution,
       cpu_cost,io_cost,temp_space,access_predicates,filter_predicates,
       plan_id,OBJECT_ALIAS,DEPTH,PROJECTION,TIME,QBLOCK_NAME
      )
select distinct hash_value,sysdate,operation,options,object_node,object_owner,object_name,
       optimizer,search_columns,id,parent_id,position,cost,cardinality,bytes,other_tag,
       partition_start,partition_stop,partition_id,other,distribution,
       cpu_cost,io_cost,temp_space,access_predicates,filter_predicates,
       :hash_value,OBJECT_ALIAS,DEPTH,PROJECTION,TIME,QBLOCK_NAME
  from v$sql_plan
 where hash_value = :hash_value
/
col piece noprint
select distinct piece,sql_text from v$sqltext where hash_value = :hash_value order by piece
/
@?/rdbms/admin/utlxplp.sql
set linesize 80
set verify on feedback on pagesize 1000

使用方法

SQL> SELECT hash_value FROM V$SQL WHERE SQL_TEXT
  2  LIKE 'SELECT * FROM SYS.SMON_SCN_TIME';
HASH_VALUE
----------
3019898357
SQL> @get_plan.sql
Enter value for hash_value: 3019898357
SELECT * FROM SYS.SMON_SCN_TIME
-----------------------------------------------------------------------------------
| Id  | Operation         | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |               |       |       |     3 (100)|          |
|   1 |  TABLE ACCESS FULL| SMON_SCN_TIME |     1 |  1163 |     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------------

补充说明
其实9i和10g/11g中得出执行计划的出入就是在plan_table表上
在9i中：plan_table表需要通过脚本创建并且授权

SQL> connect / as sysdba;
SQL> @?/rdbms/admin/utlxplan.sql;
SQL> create public synonym plan_table for plan_table; --建立同义词
SQL> grant all on plan_table to public;--授权所有用户

在10g/11g中：plan_table表系统自带，不需要创建。因为plan_table表中含有plan_id列，而得出执行计划时该列不能为空，所以上面脚本中对于10/11g数据库必须要填充plan_id值

有些朋友创建了一个非public的dblink，现在该数据库的其他用户需要去使用该dblink，在正常情况下无访问权限，需要重新建一个dblink，或者将原dblink修改为public。但是由于忘记了原dblink的目标段的密码，使得创建或者修改dblink的步骤无法进行下去。这里通过修改基表(link$)，解决该问题。

创建dblink

SQL> show user;
USER is "SYS"
SQL> create database link "xff_dblink"
  2  connect to TEST
  3  identified by "test"
  4  using '11.1.1.1:1521/mcrm';
Database link created.
SQL> select * from dba_db_links where db_link like 'XFF_DBLINK%';
OWNER DB_LINK                                     USERN HOST               CREATED
----- ------------------------------------------- ----- ------------------ --------
SYS   XFF_DBLINK.REGRESS.RDBMS.DEV.US.ORACLE.COM  TEST  11.1.1.1:1521/mcrm 29-MAR-12
SQL> select sysdate from dual@xff_dblink;
SYSDATE
---------
29-MAR-12
SQL> CONN TEST/TEST
Connected.
SQL> SELECT SYSDATE FROM DUAL@XFF_DBLINK;
SELECT SYSDATE FROM DUAL@XFF_DBLINK
                         *
ERROR at line 1:
ORA-02019: connection description for remote database not found
--该dblink不是public的，所以test用户无权访问

dblink变为public类型

SQL> CONN / AS SYSDBA
Connected.
SQL> set long 1000
SQL> select  text from dba_views where view_name='DBA_DB_LINKS';
TEXT
-------------------------------------------------------------------
select u.name, l.name, l.userid, l.host, l.ctime
from sys.link$ l, sys.user$ u
where l.owner# = u.user#
--查询出dblink相关的基表有link$和user$
SQL> desc sys.link$
 Name                          Null?    Type
 ----------------------------- -------- --------------------
 OWNER#                        NOT NULL NUMBER
 NAME                          NOT NULL VARCHAR2(128)
 CTIME                         NOT NULL DATE
 HOST                                   VARCHAR2(2000)
 USERID                                 VARCHAR2(30)
 PASSWORD                               VARCHAR2(30)
 FLAG                                   NUMBER
 AUTHUSR                                VARCHAR2(30)
 AUTHPWD                                VARCHAR2(30)
 PASSWORDX                              RAW(128)
 AUTHPWDX                               RAW(128)
SQL> select owner# from sys.link$ where name like 'XFF_DBLINK%';
    OWNER#
----------
         0
--XFF_DBLINK对应的用户标识记录在link$.owner#中
SQL> SELECT USER#,NAME FROM USER$ WHERE name in ('SYS','PUBLIC');
     USER# NAME
---------- ------------------------------
         1 PUBLIC
         0 SYS
--现link$.owner#值为0，表示该dblink所属用户为SYS，现在让该dblink变为public
--现需要让该dblink变为public，需要做的是修改link$.owner#的值为1
SQL> UPDATE LINK$ SET OWNER#=1 WHERE name like 'XFF_DBLINK%';
1 row updated.
SQL> COMMIT;
Commit complete.
--需要刷新shared_pool
SQL> ALTER SYSTEM FLUSH SHARED_POOL;
System altered.
--查看dblink所属者，已经修改为public
SQL> select owner from dba_db_links where db_link like 'XFF_DBLINK%';
OWNER
----------
PUBLIC
--测试dblink是否成功
SQL> CONN TEST/TEST
Connected.
SQL> SELECT SYSDATE FROM DUAL@XFF_DBLINK;
SYSDATE
---------
29-MAR-12

不设置sga_max_size参数

SQL> show sga;
Total System Global Area  420549952 bytes
Fixed Size                   451904 bytes
Variable Size             201326592 bytes
Database Buffers          218103808 bytes
Redo Buffers                 667648 bytes
SQL> select sum(bytes)from v$sgastat;
SUM(BYTES)
----------
 420538688
SQL> !ipcs -m
------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x9ba476a4 65536      oracle    640        440401920  65
SQL> alter system set db_cache_size=300M;
alter system set db_cache_size=300M
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-00384: Insufficient memory to grow cache

1)当sga_max_size不设置时，数据库启动时，会使用数据库默认分配sga大小为初始化值
2)当sga_max_size不设置时，不能在线扩展组件内存大小(使得sga大于当前大小)

设置sga_max_size参数

SQL>  alter system set sga_max_size=600M scope=spfile;
System altered.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area  638654020 bytes
Fixed Size                   452164 bytes
Variable Size             419430400 bytes
Database Buffers          218103808 bytes
Redo Buffers                 667648 bytes
Database mounted.
Database opened.
SQL> !ipcs -m
------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x9ba476a4 98304      oracle    640        658505728  65
SQL> select sum(bytes)from v$sgastat;
SUM(BYTES)
----------
 420538948
SQL> alter system set db_cache_size=405M;
System altered.
SQL> select 638654020/1024/1024 from dual;
638654020/1024/1024
-------------------
         609.067936
说明sga中剩余空闲内存0.01M
SQL> select 609.067936-sum(bytes)/1024/1024 from v$sgastat;
609.067936-SUM(BYTES)/1024/1024
-------------------------------
                     .010742244
继续增加组件大小
SQL> alter system set db_cache_size=416M;
System altered.
SQL> select sum(bytes)/1024/1024 from v$sgastat;
SUM(BYTES)/1024/1024
--------------------
          609.057194
SQL>  select 638654020/1024/1024 from dual;
638654020/1024/1024
-------------------
         609.067936
SQL> alter system set db_cache_size=417M;
alter system set db_cache_size=417M
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-00384: Insufficient memory to grow cache
这里显示，当db_cache_size增加到415M的时候，sga只有0.01M剩余
但是直到db_cache_size增加到417的时候才报错

1)当sga有剩余时，可以动态调整sga中的部分组件(java_pool_size不能在线设置)
2)当sga没有剩余时，如果继续增加某组件的内存，在一定的范围内，sga会自动调整其他组件大小，以实用该值增加

关于sga_max_size总结
1)如果你的系统内存比较紧张，对停机时间要求不是特别严格，那可以不设置sga_max_size参数，这样在重启数据库设置sga组件的时候，不会因忘记设置sga_max_size而导致不能正常启动
2)如果你的系统内存充足，对停机有严格限制，那建议设置一个较大的sga_max_size，后续可以根据需求动态在线调整sga部分组件

在很多时候，希望给用户最小的权限，让其访问系统数据字典，检查数据库的运行状态。这种事情在乙方的工作中非常常见。下面介绍三种方法处理这个问题
0.select any table权限
这里说明select any table不能直接访问数据字典

SQL> conn / as sysdba
Connected.
SQL> create user xff_any identified by xifenfei;
User created.
SQL> grant connect,select any table to xff_any;
Grant succeeded.
SQL> conn xff_any/xifenfei
Connected.
SQL>  select count(*) from dba_users;
 select count(*) from dba_users
                      *
ERROR at line 1:
ORA-00942: table or view does not exist
SQL> select count(*) from chf.t_xff;
  COUNT(*)
----------
      2770

select any table默认情况下，只能访问业务的表，但是不能访问数据字典的数据。所以单纯的这个属性不能满足需求。

1.SELECT ANY DICTIONARY权限

SQL> conn / as sysdba
Connected.
SQL> create user xff_DICTIONARY identified by xifenfei;
User created.
SQL> grant connect to xff_DICTIONARY;
Grant succeeded.
SQL> conn xff_DICTIONARY/xifenfei
Connected.
SQL> select count(*) from dba_users;
select count(*) from dba_users
                     *
ERROR at line 1:
ORA-00942: table or view does not exist
SQL> conn / as sysdba
Connected.
SQL> grant SELECT ANY DICTIONARY to xff_DICTIONARY;
Grant succeeded.
SQL> conn xff_DICTIONARY/xifenfei
Connected.
SQL> select count(*) from dba_users;
  COUNT(*)
----------
        32
SQL> select count(*) from chf.t_xff;
select count(*) from chf.t_xff
                         *
ERROR at line 1:
ORA-00942: table or view does not exist

这里可以看出SELECT ANY DICTIONARY权限只能访问数据字典，不能访问业务的表，访问业务的表需要另外授权

2.SELECT_CATALOG_ROLE角色

SQL> conn / as sysdba
Connected.
SQL> create user xff_CATALOG identified by xifenfei;
User created.
SQL> grant connect,SELECT_CATALOG_ROLE to xff_CATALOG;
Grant succeeded.
SQL> conn xff_CATALOG/xifenfei
Connected.
SQL> select count(*) from dba_users;
  COUNT(*)
----------
        33
SQL> select count(*) from chf.t_xff;
select count(*) from chf.t_xff
                         *
ERROR at line 1:
ORA-00942: table or view does not exist

这里可以看出SELECT_CATALOG_ROLE权限只能访问数据字典，不能访问业务的表，访问业务的表需要另外授权

3.O7_DICTIONARY_ACCESSIBILITY参数

SQL> conn / as sysdba
Connected.
SQL> create user xff_O7 identified by xifenfei;
User created.
SQL> grant connect to xff_o7;
Grant succeeded.
SQL> alter system set O7_DICTIONARY_ACCESSIBILITY=true;
alter system set O7_DICTIONARY_ACCESSIBILITY=true
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified
SQL> alter system set O7_DICTIONARY_ACCESSIBILITY=true scope=spfile;
System altered.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area  368263168 bytes
Fixed Size                  1345016 bytes
Variable Size             306186760 bytes
Database Buffers           54525952 bytes
Redo Buffers                6205440 bytes
Database mounted.
Database opened.
SQL> conn xff_o7/xifenfei
Connected.
SQL> select count(*) from dba_users;
  COUNT(*)
----------
        34

这里通过O7_DICTIONARY_ACCESSIBILITY和SELECT ANY TABLE权限，实现访问业务数据和数据字典